In traditional linear audio amplifiers, DC bias current makes up a large portion of the total power consumption. As a result, the power efficiency is typically limited to around 60%. In contrast, Class-D amplifiers use the much more efficient switch-mode operation and power efficiency as high as 80-90% is readily obtained. Better efficiency means lower heat dissipation. As a result, simple, low cost and compact thermal management systems are usually sufficient. This is one of the main reasons Class-D is often preferred over linear amplifiers. However, Class-D audio amplifiers are sensitive to modulator non-linearity, power stage non-idealities, and power supply noise which make achieving low distortion difficult.
Total Harmonic Distortion plus Noise (THD+N) measures the distortion in the audio signal produced by an amplifier. The lower the THD+N value the better, because a lower THD+N indicates a lower level of distortion and therefore higher audio fidelity. Low distortion in Class-D audio amplifiers is difficult to achieve without the use of feedback.
In analog Class-D amplifiers, the power stage gating signal is produced using analog signal processing. A typical high performance analog Class-D amplifier consists of a power output stage, an analog modulator, a feedback network, and an audio Digital to Analog Converter (DAC). The fidelity of the amplifier is limited by the distortion in the power stage and the modulator. Therefore, to obtain better audio performance, a distortion suppression feedback loop is placed around them. In such a system, the audio DAC is required to have very low THD+N.
In a digital Class-D amplifier, the power stage gating signal is generated using digital signal processing. A typical high performance digital Class-D amplifier consists of a power stage, a digital modulator and a feedback loop. One advantage of using digital signal processing is that the modulator performance is largely unaffected by circuit non-idealities and therefore very low distortion can be achieved. To improve audio fidelity, a distortion suppression feedback loop is placed around the power stage. Since no high performance analog circuits are required, significant cost saving can be realized.
In surveys of high performance Class-D amplifiers, open-loop analog amplifiers typically perform worse than their digital counter parts. However, when feedback is employed, sub 0.002% THD+N performance has been achieved. In contrast, digital designs improve only slightly, with feedback and THD+N values remaining large in comparison. Therefore, the biggest challenge in digital Class-D is breaking the performance limitation, and doing so without significant cost overhead.